1. Field of the Invention
The present invention relates to a data transmission method and apparatus for use in a digital transmission network, and in particular to such a transmission method and apparatus for use in a hierarchically organized transmission network.
2. Description of the Prior Art
Several methods for bus transmission of digital data in networks organized based on a prescribed hierarchy of the connected network nodes and assemblies are known. Within such so-called master-slave systems, a higher ranking unit (master) supervises the transmission paths and forwards instructions to the lower-ranking units (slaves) which execute these instructions and which react to the instructions with acknowledgement signals.
CCITT Recommendation X.25, which regulates data exchange on communication links, is a transmission rule frequently employed in public networks. In conformity with this rule, data to be transmitted are arranged in serial bit sequence, and together with address bits are packaged into information units in a frame consisting of control and check bits.
A telephone switching system represents an applied example of a digital data transmission network. Such systems generally include a central switching center having an arithmetic unit serving as the master, and a plurality centralized (peripheral) concentrators dependent on the master, with terminals connected thereto. In addition to the actual useful information which is transmitted between the terminals, so-called signaling data must additionally be transmitted between the terminal and switching system such as, for example, busy signals, answering signals, ringing signals, selection signals, and billing information. A further bus on which the signaling data are communicated in packet form is therefore provided in addition to a bus for the useful information between the concentrators and the central arithmetic unit.
Essential auxiliary functions in the execution of data transmission according to the above-described master-slave principle are undertaken by check or control modules with which all lower ranking units are equipped. The functions of such modules are described in the typical transmission procedure described in brief below, commonly utilized in conventional units for the exchange of signaling packets between local concentrators and a central arithmetic unit (processor). The master first successively enables the slaves to transmit data. The check or control module of the respectively addressed slave unit subsequently uses an interupt request to cause the allocated master to evaluate the command. After a prescribed reaction time dependent, for example, on the time necessary to ready the data to be transmitted, the master initiates transmission operation of the slave unit. The data are transmitted to the master. Further details regarding conventional operation of such a controller module are described, for example, in INTEL, Comp. Data Catalog, January 1981, pages 8-163 through 8-175.
If the network is used in instances wherein a relatively small amount of data arises, a large portion of the available processing time of the master is unnecessarily dedicated to the execution of these method steps as a result of this transmission protocol because the address controller unit reports every inquiry per interupt request to the processor, and the communication link is consequently inhibited during this time. It has been statistically demonstrated, for example, that given a telephone switching system comprising a plurality of subscriber terminals, only a small portion of the subscribers respectively use or wish to use the communication link simultaneously. Particularly in such cases, known transmission methods therefore have the disadvantage that a relatively time-intensive request/acknowledgement procedure must be cyclicly executed with all subscribers, even though no data are offered for transmission or no transmission is requested. Within a prescribed time cycle, however, the number of lower ranking units with which the central unit can communicate is dependent on the duration of the individual data exchange procedures. Such time redundancy thus reduces the number of possible communication partners within a network.